1. Field of The Invention
The present invention is broadly concerned with NO.sub.x abatement, especially with NO.sub.x abatement in the exhaust gas of lean-burn internal combustion engines. More specifically, the present invention is concerned with a novel composition comprising a combination of a NO.sub.x trap (sorbent material) and a NO.sub.x abatement catalyst, and to a method of using the same.
2. Related Art
It is well-known in the art to utilize catalyst compositions, such as those commonly referred to as three-way conversion catalysts ("TWC catalysts") to treat the exhaust gases of internal combustion engines. Such catalysts have been found to successfully promote both the oxidation of unburned hydrocarbons ("HC") and carbon monoxide ("CO") and the reduction of nitrogen oxides ("NO.sub.x ") in the exhaust gas, provided that the engine is operated at or close to stoichiometric air/fuel conditions. However, many present day engines, especially gasoline-fueled engines used for passenger automobiles and the like, are designed to operate under lean conditions at least part of the time as a fuel economy measure. That is, the ratio of air to fuel in the combustion mixture supplied to the engine is maintained above the stoichiometric ratio so that the resulting exhaust gases are "lean", i.e., the exhaust gases are relatively high in oxygen content and relatively low in reductants content, e.g., HC, CO and/or hydrogen.
Although lean burn engines provide enhanced fuel economy, they have the disadvantage that conventional TWC catalysts cannot adequately abate the NO.sub.x component of pollutants in the gas stream. The art has devoted some effort to developing catalysts specifically designed to treat the exhaust of lean burn engines. One such effort is described in the publication Environmental Catalysis For A Better World And Life, Proceedings of the 1st World Congress at Pisa, Italy, May 1-5, 1995, published by the Societa Chimica Italiana of Rome, Italy. At pages 45-48 of the publication, there is an article entitled "The New Concept 3-Way Catalyst For Automotive Lean-Burn Engine Storage and Reduction Catalyst", by Takahashi et al (below referred to as "the Takahashi et al paper"). This article discloses the preparation of catalysts by impregnating precious metals, mainly platinum, and various alkaline and alkaline earth metal oxides, mainly barium oxide, and rare earth oxides on supports, mainly alumina. These catalysts were employed for NO.sub.x purification of actual and simulated exhaust gases. At page 47 of the article, there is both shown graphically in FIG. 5 and discussed in paragraph 3.1.1., "NO.sub.x Storage Mechanism", the concept of employing NO.sub.x storage compounds in conjunction with the catalytic (platinum) component, with both the NO.sub.x storage compounds and the catalytic component dispersed on a common support material. It is stated that "When precious metals were separated from the NO.sub.x storage compounds on these catalysts, the NO.sub.x storage amount drastically decreased." Paragraph 3.1.1. therefore teaches that the NO.sub.x storage compound and the platinum catalyst should be intimately admixed by being disposed on the same increment of, e.g., alumina, support as described in the catalyst preparation procedure described in paragraph 2 (page 45) and illustrated in FIG. 5.
In paragraph 3.1, page 46, testing of the catalysts is described as being carried out with simulated exhaust gases alternately under oxidizing and reducing conditions. The conclusion is drawn on the last sentence on page 46, that NO.sub.x was stored in the catalyst under oxidizing conditions and that the stored NO.sub.x was then reduced to nitrogen under stoichiometric and reducing conditions.
SAE paper 950809 published by the Society of Automotive Engineers, Inc., Warrendale, Pa., and entitled Development of New Concept Three-Way Catalyst for Automotive Lean-Burn Engines, by Naoto Miyoshi et al, was delivered at the International Congress and Exposition, Detroit, Mich., Feb. 27-Mar. 2, 1995. This paper, referred to below as "the SAE paper", which has authors in common with the above-mentioned Takahashi et al paper, contains a disclosure which is substantially the same as, but is more detailed than, that of the Takahashi et al paper. At page 123 of the SAE paper a correlation between basicity of the elements used as NO.sub.x storage material and NO.sub.x storage capacity is discussed as is NO.sub.x adsorption by complex oxides such as YBa.sub.2 Cu.sub.3 Oy and CuO--BaO. The discussion of mixed oxides is made with reference to items 6 and 7 of the references cited at the end of the SAE paper. The references are "Uptake of NO gas by YBa.sub.2 Cu.sub.3 Oy" by K. Tabata et al, Chem. Lett., 1988, pp. 799-802 and "NO removal by adsorption into BaO--CuO binary oxides" by M. Machida et al, J. Chem. Soc. Chem. Commum. 1990, pp. 1165-1166. At page 125 of the SAE paper, there is a comparison of two catalysts. One catalyst comprises (a) noble metal impregnated on one increment of alumina and the NO.sub.x storage component impregnated on alumina, and the other catalyst comprises (b) both noble metal and the storage component impregnated on the same increment of alumina. The SAE paper states that catalyst (a) stored a very small amount of NO.sub.x as compared to catalyst (b) and concludes that the "storage amount increases when the noble metal is in the proximity of the storage component".
U.S. Pat. No. 5,202,300, "Catalyst For Purification of Exhaust Gas", issued on Apr. 13, 1993, to M. Funabiki et al, discloses a catalyst composition comprising a refractory support having deposited thereon an active layer containing a palladium and rhodium catalytic metal component dispersed on alumina, a cerium compound, a strontium compound, and a zirconium compound. (See the Abstract.) This patent discloses the preparation of the catalyst by impregnating alumina particles with palladium and rhodium and then combining the impregnated alumina with compounds such as cerium nitrate, strontium acetate hemi-hydrate and zirconyl acetate together with acetic acid and deionized water in a ball mill, and crushing and mixing the mixture to provide a slurry which is applied to a monolithic, cylindrical cordierite carrier. See columns 3-5 of the Funabiki et al Patent, especially Example 1 which discloses that the strontium compound is solubilized in the acidified water and impregnated into the alumina support particles together with the palladium and rhodium catalytic metal components. Example 1 as Funabiki et al, like the Takahashi et al and SAE papers referred to above, impregnates both the alkaline earth metal compound (a strontium compound in the case of Funabiki et al), and the palladium and rhodium catalytic metal components into the same increment of (alumina) support material. However, Example 2 of Funabiki et al substitutes strontium hydroxide for the strontium acetate and the limited solubility of strontium hydroxide suggests that it remains in bulk form in the finished product, admixed with the palladium and rhodium-impregnated alumina.